The invention relates to a cardiac pacemaker according to the preamble of claim 1.
The publication "Biomed. Technik [Biomedical Technology], 34 (1989), pages 177-184"0 discloses a PEP [pre-ejection period] controlled cardiac pacemaker. The term PEP is understood to mean the interval from the beginning of chamber depolarization to the beginning of the ejection phase. The implantable stimulation system presented there, which includes PEP controlled physiological frequency regulation, is based on the relationship between the PEP and the chronotropic heart rate. The shortening of the PEP can be converted to an increase in frequency. In the case of a decrease of the adrenergic effect, both intervals are extended. The solution disclosed there is based on the right ventricular intracardial detection of a time interval from the impedance curve obtained between the electrode tip and the pacemaker housing which coincides well with the left ventricular PEP.
The "ideal" physiological parameter for a controlled adaptation of the stimulation rate of an artificial cardiac pacemaker reflects physical or emotional stress but is independent of the cardiac frequency. The frequency can be determined with the aid of a truth table or as a functional relationship from the momentarily measured value of the physiological parameter. The tensioning time, the PEP, has these "ideal" characteristics. In a constant stress situation, the PEP is independent of the stimulation rate and with a constant stimulation rate, it is an unequivocal function of stress.
If, however, the PEP is employed as a physiological parameter for the controlled adaptation of the stimulation rate of an artificial cardiac pacemaker, the purely static stress on the basis of the volume of blood involved must be considered. This static stress differs because of the distribution of the blood in the artery system in the standing or recumbent state. Although the above-mentioned literature citation depicts, at page 183, FIG. 13, an algorithm which, in the longer duration rest case, is based on a patient in a recumbent position, this algorithm is inaccurate and not suitable to ensure quick adaptation of the heart rate to actual stress conditions.